A water and oil proofing agent has been proposed which contains a fluorocopolymer (crystallizable polymer) prepared by using an Rf group-containing monomer which forms a homopolymer having a melting point of crystallites attributable to polyfluoroalkyl groups (hereinafter referred to as Rf groups).
As such a fluorocopolymer, e.g. the following Patent Document 1 proposes a quaternary fluorocopolymer comprising polymerized units based on a perfluoroalkyl group (hereinafter referred to as an RF group)-containing acrylate, polymerized units based on stearyl (meth)acrylate, polymerized units based on 2-chloroethyl vinyl ether and polymerized units based on N-methylol (meth) acrylamide.
Further, the following Patent Document 2 proposes a quaternary fluorocopolymer comprising polymerized units based on an RF group-containing acrylate, polymerized units based on a (meth)acrylate containing an alkyl group or the like, polymerized units based on an acrylate containing a polyoxyalkylene group and polymerized units based on an acrylate containing a dialkylamino group.
Further, the following Patent Document 3 proposes a copolymer essentially comprising polymerized units based on an RF group-containing acrylate, polymerized units of an α,β-ethylenic unsaturated monomer having a carboxyl group and polymerized units of another α,β-ethylenic unsaturated monomer.
Here, most of perfluoroalkyl groups used in conventional fluorinated water and oil proofing agents have at least 8 carbon atoms. In recent years, from the viewpoint of the influence to the environment, compounds having perfluoroalkyl groups are recommended to have less than 8 carbon atoms in their perfluoroalkyl groups.
In this respect, the following Patent Document 4 discloses a fluoropolymer obtained by polymerizing a (meth)acrylate monomer containing an RF group having from 1 to 6 carbon atoms, with a nitrogen-containing monomer.    Patent Document 1: JP-A-10-237133    Patent Document 2: JP-A-5-271351    Patent Document 3: JP-A-5-17538    Patent Document 4: WO2005/100421